1. Field of the Invention
The present invention relates to a method for fabricating a complementary metal-oxide semiconductor (CMOS) image sensor, and more particularly, to a CMOS image sensor and a method for fabricating the same in which a dark current is prevented from occurring, thereby improving performance of the image sensor.
2. Discussion of the Related Art
Generally, an image sensor is a semiconductor device that converts optical images to electrical signals. Image sensors can be classified as a charge coupled device (CCD) image sensor or a CMOS image sensor.
The CCD includes a plurality of photodiodes arranged in a matrix to convert optical signals to electrical signals, a plurality of vertical charge coupled devices (VCCDs) formed between the photodiodes to transfer charges generated by the respective photodiodes in a vertical direction, a plurality of horizontal charge coupled devices (HCCDs) for transferring the charges transferred by the VCCDs in a horizontal direction, and a sensing amplifier for sensing the charges transferred in a horizontal direction to output electrical signals.
However, the aforementioned CCD has drawbacks in its fabrication process because of a complicated driving mode, high power consumption, and multistage photolithographic processes. Moreover, it is difficult for a signal processing circuit and an analog-to-digital converter to integrate on a chip of the CCD. Moreover, a slim product size is difficult to obtain.
The CMOS image sensor attempts to overcome the drawbacks of the CCD. The CMOS image sensor employs a switching mode that sequentially detects outputs of unit pixels using MOS transistors by forming the MOS transistors corresponding to the number of the unit pixels on a semiconductor substrate. The CMOS technology uses a control circuit and a signal processing circuit as peripheral circuits. The CMOS image sensor sequentially detects electrical signals of each unit pixel using the switching mode to display images by forming photodiodes and MOS transistors in the unit pixels.
The CMOS image sensor can have a low power consumption and a simple fabrication process due to the relatively small number of photolithographic process steps. Further, since the CMOS image sensor allows a control circuit, a signal processing circuit and an analog-to-digital converter to be integrated on one chip, a slim sized product can be obtained. Therefore, the CMOS image sensor is widely used for various applications, such as digital cameras and digital video cameras.
A conventional CMOS image sensor will be described with reference to FIG. 1 to FIG. 2.
FIG. 1 is a layout illustrating a unit pixel of a 3T type CMOS image sensor including three transistors. As shown in FIG. 1, the unit pixel of the 3T type CMOS image sensor includes a photodiode 20 and a transistor region 10. The transistor region 10 includes three transistors, i.e., a reset transistor Rx 120 for resetting optical charges collected in the photodiode, a drive transistor Dx 130 serving as a source follower buffer amplifier, and a select transistor Sx 140 for switching addressing.
FIG. 2 is a sectional view taken along line II—II of FIG. 1. As shown in FIG. 2, a P− type epitaxial layer 101 is formed on a P++ type semiconductor substrate 100, and a device isolation film 103 is formed in a device isolation region formed on the a P− type epitaxial layer 101. A gate 123 is formed above a portion of the epitaxial layer 101 by interposing a gate insulating film 121 therebetween. An insulating spacer 125 is formed at both sides of the gate 123. An N− type diffusion region 131 is formed on an active region of the epitaxial layer 101 defined by the device isolation film 103, and a P° type diffusion region 132 is formed on the N− type diffusion region 131. A heavily doped N type diffusion region (N+) and a lightly doped N type diffusion region (N−) are formed. The heavily doped N type diffusion region (N+) and the lightly doped N type diffusion region (N−) serve as source and drain regions S/D.
However, the conventional CMOS image sensor has drawbacks in that the sensor and its charge storage ability performance deteriorates are a result of dark current.
Dark current refers to current generated by electrons moving from the photodiode to another region such that light does not enter the photodiode. The dark current is generally caused by various defects or dangling bonds generated in a portion adjacent to the surface of the semiconductor substrate, a boundary portion between the device isolation film and the P° type diffusion region, a boundary portion between the device isolation film and the N− type diffusion region, a boundary portion between the P° type diffusion region and the N− type diffusion region, the P° type diffusion region, and the N− type diffusion region. The dark current may cause serious problems under low illumination conditions.
Therefore, in the conventional CMOS image sensor, the P° type diffusion region is formed on the surface of the photodiode to reduce the dark current, particularly the dark current generated in the portion adjacent to the surface of the silicon substrate. However, the conventional CMOS image sensor is affected greatly by the dark current generated in the boundary portion between the device isolation film and the P° type diffusion region and the boundary portion between the device isolation film and the N− type diffusion region.
In more detail, as shown in FIG. 2, a photoresist pattern (not shown) is formed on the semiconductor substrate 100 as an ion implantation mask layer to form the N− type diffusion region 131 and the P° type diffusion region 132. At this time, the entire active region for the photodiode is exposed from an opening of the photoresist pattern. In this state, if impurity ions for the N− type diffusion region 131 and the P° type diffusion region 132 are implanted into the active region of the photodiode, the impurity ions are also implanted into the boundary portion between the active region and the device isolation film 103.
Therefore, the boundary portion between the device isolation film 103 and the N− type diffusion region 131, and the boundary portion between the device isolation film 103 and the P° type diffusion region 132 are damaged by ion implantation of the impurity ions, thereby causing defects. The defects cause electron-hole carriers and recombination of the electrons. As a result, the leakage current of the photodiode is increased, and the dark current of the CMOS image sensor is increased.
As described above, the conventional CMOS image sensor has a structure in which the impurity ions are implanted into the boundary portion between the device isolation film and the active region for the photodiode during ion implantation of the impurity ions for the formation of the diffusion regions of the photodiode. As a result, in the conventional CMOS image sensor, it is difficult to prevent an increase in dark current generated in the boundary portion between the device isolation film and the active region for the photodiode. This limits the performance of the CMOS image sensor.